1. Field of the Invention
This invention relates to a system of aerobically treating waste sludge. More particularly, the invention is directed to an orbital system and method for treating by digestion and thickening bacterial microorganism-containing sludge wasted from an industrial or municipal wastewater treatment process and apparatus.
2. State of the Art
In a wastewater process employing the activated sludge process waste water impurities including domestic wastes, sugars, proteins, carbohydrates and other nitrogen-containing materials are decomposable by microorganisms, as is well known in the art. As the impurities are decomposed a sludge of settled material and microorganisms is wasted from the process either on a continuous or non-continuous basis. The purpose of sludge wasting is to keep solids from building up in the system. Sludge from the process is normally transported by pumping to a digester for treatment prior to landfill or other disposal so as to reduce the volatile organic content of the sludge, reduce the sludge volume, reduce the pathogenic organisms present in the sludge, reduce its odor potential and improve sludge dewaterability, and for other reasons of lesser importance. Various prior art types of digesters and various digestion and stabilization processes have been proposed and used.
The present invention is directed to an apparatus and method involving improvements in aerobic digestion. Current practice for aerobic digestion is to first transport the waste sludge generated in a secondary activated sludge system into a separate adjacent thickening tank of the gravity or other type and then transport the thickened sludge to a separate aerated digester for 10-30 days. Biological activity in the tank digests the sludge over that time.
In addition, these digesters provide sludge holding capability for periods of time when the ultimate disposal method is not available (i.e., the sludge truck is broken or the landfill is closed, etc.) and they provide a means of thickening the sludge, called decanting. Decanting is simply allowing the sludge to settle by turning off the aeration for a period of time until a clear liquid layer forms on top of the sludge. This clear liquid is then removed from the digester tank which leaves behind a thicker sludge having less volume. Decanting can be a fill and draw operation where the water level in the tank varies or the clear liquid can be displaced by sludge being fed to the tank. There are also some floating weir decanters in use. In some cases the sludge is thickened in a separate thickener after digestion. The purpose of this is again to thicken the sludge and reduce its volume.
Aerobic digestion is described more fully on pages 5-20 to 5-28 of U.S. Environmental Protection Agency (EPA) Publication 62511-74-006 entitled "Process Design Manual for Sludge Treatment and Disposal" dated October 1974. As pointed out on page 5-23 aerobic digestion has disadvantages which may include high operating costs and poor dewatering characteristics on vacuum filters. A relatively long detention time of 10-30 days or more is necessary to break down the organic matter and control pathogenic organisms and is common practice. In small treatment systems (0.1 mgd) the digestion tank can also act as a sludge thickening unit. The current practice is to separately pump air or oxygen into the digester through diffuser pipes in the liquid-solids mass in the tank in an amount of from about 20-60 cfm/1000 cubic feet, dependent on the presence of primary and/or excess activated sludge, to keep the solids in suspension and maintain a dissolved oxygen (DO) between 1-2 mg/l (page 5-26).
It has been common for some years to use a so-called orbital oxidation system or oxidation ditch to biologically treat wastewater. Such systems are exemplified by the U.S. patent to Klein 3,510,110 where sidewalls and a center partition form a race track-like essentially closed circuit with a surface aerator positioned adjacent to one end of the partition to both aerate the mixed liquor in the tank and to provide the motive force for circulation of the mixed liquor around the circuit. Horizontal brush-type aerators, jet-type aerators or rotating disc-type aerators have also been employed in activated sludge sewage orbital purification processes. As seen in Stensel et al., (4,303,516) clarifiers, (termed intraclarifiers) which have a sludge thickening function have been placed into a channel of an orbital ditch and function to decant off clarified liquid from a portion of the overall channel mixed liquor flow introduced into the clarifier and to allow settled sludge to be returned to the channel from the intraclarifier.
Further, it has been known to use an oxidation ditch as an aerobic digester per se. Reference is also made to McKinney, U.S. Pat. No. 3,462,360 which shows a loop-type secondary aerobic treatment apparatus in which liquid waste is treated and a separate settling basin provided defining a quiescent liquid zone for conducting tertiary treatment. Settled algae may be returned from the settling basin to the loop or withdrawn from the facility.